The Fab fragment of a monoclonal antibody (mAb41.4) capable of inhibiting specific transcription factor binding to DNA is being studied by X-ray crystallography. Fab4 inhibits binding of ATF-1 to the CRE DNA sequence. It is critical to this particular project to obtain a high-resolution structure. Crystals have been very sensitive to x-rays at room temperature, so cryocollection techniques are very important. An initial partial data set has produced promising results but the sensitive nature of the crystals have prevented further progress. Greater completeness and higher resolution would clarify the molecular recognition of the Fab4 for the peptides.